The present disclosure relates to systems and methods useful for non-destructive evaluation (NDE) of molds and crucibles used in investment casting processes, including, without limitation, for producing aircraft engines, land-based turbine engines, and the like.
Molds and crucibles are commonly used in making components and parts for high performance devices such as aircraft and land-based turbine engines. Uniformity and structural integrity of such molds and crucibles are important in maintaining quality control in investment casting processes. Control of the full volume geometry and the wall thickness of investment casting molds and crucibles is important to ensure structural integrity of the mold or crucible and to prevent leaks occur during melting and casting. X-ray technology has been used to analyze the structural integrity of molds and crucibles. However, such X-ray techniques require highly specialized personnel and equipment, and generally more time-consuming than is permissible for a manufacturing environment. Therefore, there remains a need for practical techniques for inspecting the full volume geometry or wall thickness of a mold or crucible in a non-destructive manner prior to melting and casting.
The thickness of the wall is important because it has a major effect on the strength of the ceramic shell component. For example, if the thickness of the wall of the ceramic shell component falls below a minimum threshold value, the strength of the component falls below the minimum acceptable strength, and the liquid metal can leak from the crucible or mold. In some cases this leakage can result in a casting failure. Accordingly, wall thickness is a critical parameter of melting crucibles and casting molds, as thickness below a specification limit increase the likelihood of part loss during the casting operation.
There are current practices to check the wall thickness of crucibles. One current practice is to sample crucibles on a low-frequency basis. The measurement process involves installing the crucible into a fixture with point contacts on the internal face. Measurements are made with a dial indicator on or about points on the surface. After these measurements are made, the crucible must be discarded because the point-contacts damage the contacting surface of the crucible.
Induction melting generally involves heating a metal in a crucible made from a non-conductive refractory alloy oxide until the charge of metal within the crucible is melted to liquid form (see, e.g., U.S. Pat. No. 8,048,365 to Bewlay et al.). When melting highly reactive metals such as titanium or titanium alloys, vacuum induction melting using cold wall or graphite crucibles can be employed instead of oxide based ceramic crucibles. Difficulties can arise when melting highly reactive alloys, such as titanium alloys, as a result of the reactivity of the elements in the alloy at the temperatures used for melting. While most induction melting systems use refractory oxides for crucibles in the induction furnace, alloys such as titanium aluminide are so highly reactive that they can attack the refractory oxides present in the crucible and the titanium alloy becomes contaminated. For example, ceramic crucibles are typically avoided because the highly reactive alloys can react with the crucible and contaminate the alloy, with oxygen for example. Similarly, if graphite crucibles are employed, both the titanium and titanium aluminide based alloys can dissolve carbon from the crucible into the titanium alloy, thereby resulting in contamination and loss of mechanical properties of the resulting article.
Cold crucible melting offers advantages for processing highly reactive alloys, but it also has a number of technical and economic limitations, including low superheat, yield losses due to skull formation, high power requirements, and limited melt capacity. Any reaction between the molten alloy and the crucible will tend to deteriorate the properties of the casting. The deterioration can be as simple as poor surface finish due to gas bubbles, or worse, the chemistry, microstructure, and properties of the casting can be compromised.
Accordingly, it is important to have inspection techniques in place to ensure the production of high quality crucibles for use in melting the metal or metal alloy. In one example, the inspection techniques are applied to ceramic crucibles used for highly reactive alloys, as the ceramic crucibles are less susceptible to contamination and pose fewer technical and economic limitations than current techniques. For example, rather than destructive sampling based inspection techniques, there is a need for inspection technologies that are non-destructive so as to preserve yields and that are effective to enable fast and efficient inspection.
The present systems and methods are directed to overcoming these and other deficiencies in the art.